1. Field of the Invention
The present invention relates to a gadolinium oxysulfide phosphor for digital radiography which exhibits mainly green emission of high luminance with a short afterglow, under excitation with radiation such as X-rays or xcex3-rays, a high sensitive radiographic image conversion screen mainly for digital radiography, employing the phosphor for a fluorescent layer, and a radiograph-forming device which converts a radiographic image of an object into a visible image having a favorable image quality.
2. Discussion of Background
When a radiograph of an object is taken with a purpose of medical diagnosis or an industrial non-destructive inspection, a radiographic image conversion screen (hereinafter referred to as xe2x80x9cimage conversion screenxe2x80x9d comprising a support of e.g. paper or a plastic and a fluorescent layer comprising a phosphor which emits luminescence under excitation with ionizing radiation such as X-rays, xcex1-rays or xcex3-rays, particularly X-rays (hereinafter referred to as xe2x80x9ca phosphor for X-ray excitationxe2x80x9d) is used. The image conversion screen is used in contact with a radiographic film (film). The object is irradiated with radiation, and transmitted radiation reach the film via the image conversion screen to form a radiograph. Particularly the image conversion screen used when a radiograph is taken by using a film, is called a radiation intensifying screen (intensifying screen).
In recent years, instead of a method of obtaining a radiograph by photography by analogue system utilizing an intensifying screen/film system wherein an intensifying screen and a film are combined, so-called digital radiography (hereinafter referred to simply as DR) has been used practically, wherein a radiograph of an object formed on an image conversion screen is photoelectrically detected by using a photoelectric conversion element such as a photodiode, a photomultiplier (PM), a CCD photosensor or a CCD camera and converted into digital signals, which are subjected to image processing electrically and converted into a visible image again.
When a radiograph is taken by means of the intensifying screen/film system, a phosphor to be used for the intensifying screen is selected depending upon the relation with properties such as spectral sensitivity of a film on which the radiograph is formed and fixed. Whereas, in a case where a radiograph is taken by means of DR employing an image conversion screen/photoelectric conversion element system, the consistency with the photoelectric conversion element for DR is important. Namely, it is important to select a phosphor which generates luminescence of high luminance in a wavelength region corresponding to the spectral sensitivity of the photoelectric conversion element for DR.
When a radiograph is taken by means of DR, a phosphor to be used for the image conversion screen for DR is required to be highly sensitive and provide a shorter afterglow (persistence), with a purpose of decreasing the exposure amount of a patient as an object and decreasing a noise in radiograph-forming system. However, as the phosphor for X-ray excitation to be used for the fluorescent layer of the image conversion screen for DR, a phosphor for an intensifying screen has been used. Gadolinium oxysulfide (Gd2O2S:Tb) phosphor activated by terbium (Tb) is one of typical phosphors for intensifying screens (JP-B-55-25411). When an image conversion screen employing this phosphor for a fluorescent layer is used for DR, no adequate sensitivity in radiograph-forming system or reduction of afterglow can be achieved, and their improvements have been desired.
On the other hand, with regard to a Tb-activated oxysulfide type phosphor to be used for a cathode ray tube for display, it has been proposed to co-activate an oxysulfide of e.g. Y, Gd, La or Lu by Tb and Dy to suppress decrease in brightness due to current saturation at a high current density region when the phosphor is irradiated with electron rays having a high current density, thereby to improve emission luminance (JP-A-57-141482).
Further, as a Tb-activated oxysulfide type phosphor to be used for a cathode ray tube for display, a phosphor obtained by incorporating a rare earth element such as Tb, Pr, Dy or Tm and Ce simultaneously into an oxysulfide of e.g. Y, Gd, La or Lu, which can suppress deterioration of emission luminance due to burning of the phosphor when the phosphor is used for a fluorescent layer of a cathode ray tube for display and operated under irradiation with electron rays having a high current density for a long period of time, has been proposed (JP-A-62-79284). However, no phosphor containing Tb, Dy and Ce simultaneously is specifically disclosed, and no emission properties under excitation with X-rays are disclosed either.
Further, as confirmed by the present inventors as described hereinafter, when an intensifying screen employing such a phosphor comprising a Gd2O2S:Tb phosphor co-activated by Dy for a fluorescent layer is used in combination with an orthochromatic (ortho) type film to take a radiograph, the speed decreases contrary to expectation, as compared with an intensifying screen employing a conventional Gd2O2S:Tb phosphor not co-activated by Dy for a fluorescent layer. Namely, with respect to the phosphor comprising Gd2O2S:Tb co-activated by Dy, it is unclear whether co-activation by Dy contributes to improvement of emission luminance under excitation with X-rays, in a phosphor to be used for an image conversion screen for DR wherein an object is irradiated with X-rays to form a radiograph, which is detected by a photodetector such as a photodiode, apart from a case of irradiation with electron rays having a high current density.
Under these circumstances, it is an object of the present invention to overcome the above problems and to provide a Gd2O2S:Tb type phosphor for DR, which further improves emission luminance under excitation with X-rays, which shortens an afterglow and which has a consistency with a detector for DR. Another object of the present invention is to provide an image conversion screen for DR, which has a high sensitivity and which is less susceptible to deterioration in image quality due to influence of e.g. an afterimage, and a radiograph-forming device capable of forming a radiograph with a high image quality.
The present inventors have produced various Gd2O2S:Tb type phosphors wherein a second or a third element is added to a Tb activator for co-activation, which are irradiated with X-rays, and the emission is detected with a photoelectric conversion element for DR such as a photodiode, to study the effect of improving emission luminance by the above co-activator and afterglow properties in detail. As a result, they have succeeded in providing a Gd2O2S:Tb type phosphor which has a high sensitivity, which is less susceptible to deterioration in image quality due to influences of e.g. afterglow and which is thereby suitable for an image conversion screen for DR, by co-activating Gd2O2S:Tb by specific amounts of Dy and Ce, or by further incorporating a specific amount of Zn thereinto, and providing a high sensitive image conversion screen and a radiograph-forming device. Further, they have succeeded in providing an image conversion screen and a radiograph-forming device, having a higher sensitivity, by employing such a construction that a plurality of fluorescent layers are used for the image conversion screen, and a fluorescent layer comprising a phosphor in the form of fine particles having specific particle sizes is disposed on the support side. The construction of the present invention will be described below.
(1) A gadolinium oxysulfide phosphor for DR represented by the following compositional formula, which mainly emits green luminescence under excitation with radiation:
(Gd1xe2x88x92xxe2x88x92yxe2x88x92z,Tbx,Dyy,Cez)2O2S 
wherein x, y and z are numbers which satisfy 1.2xc3x9710xe2x88x923xe2x89xa6xxe2x89xa61.9xc3x9710xe2x88x922, 5xc3x9710xe2x88x924xe2x89xa6yxe2x89xa61.9xc3x9710xe2x88x922 and 10xe2x88x928xe2x89xa6zxe2x89xa68xc3x9710xe2x88x927, respectively.
(2) The gadolinium oxysulfide phosphor for DR according to the above (1), wherein x, y and z are numbers which satisfy 2xc3x9710xe2x88x923xe2x89xa6xxe2x89xa67xc3x9710xe2x88x923, 1.8xc3x9710xe2x88x923xe2x89xa6yxe2x89xa61.4xc3x9710xe2x88x922 and 5xc3x9710xe2x88x928xe2x89xa6zxe2x89xa64xc3x9710xe2x88x927, respectively.
(3) The gadolinium oxysulfide phosphor for DR according the above (1) or (2), which contains zinc (Zn).
(4) The gadolinium oxysulfide phosphor for DR according to the above (3), wherein the content of zinc (Zn) is from 10 to 100 ppm.
(5) The gadolinium oxysulfide phosphor for DR according to the above (4), wherein the content of zinc (Zn) is from 20 to 70 ppm.
(6) The gadolinium oxysulfide phosphor for DR according to any one of the above (1) to (5), which has an average particle size of from 1 to 5 xcexcm.
(7) The gadolinium oxysulfide phosphor for DR according to the above (6), which has an average particle size of from 2 to 4 xcexcm.
(8) The gadolinium oxysulfide phosphor for DR according to any one of the above (1) to (7), wherein the maximum peak wavelength of the emission spectrum of the phosphor is within a wavelength range of from 520 to 580 nm.
(9) A radiographic image conversion screen which comprises a support and a fluorescent layer made of a mixture of a binder and a phosphor formed on the support, wherein the phosphor is a gadolinium oxysulfide phosphor for DR as defined in any one of the above (1) to (8).
(10) The radiographic image conversion screen according to the above (9), which comprises a plurality of the fluorescent layers.
(11) The radiographic image conversion screen according to the above (10), wherein at least the fluorescent layer in contact with the support comprises the gadolinium oxysulfide phosphor for DR as defined in the above (6) or (7).
(12) The radiographic image conversion screen according to any one of the above (9) to (11), wherein a protective layer is formed on the fluorescent layer.
(13) A radiograph-forming device, comprising at least a radiographic image conversion screen which absorbs radiation transmitted through an object and forms a fluorescence image of the object, a plurality of photoelectric conversion elements disposed on each of regions on the screen two-dimensionally and finely divided in a matrix, a processing means which processes disposition signals and detections signals from each of the photoelectric conversion elements corresponding to the fluorescent image formed by the radiation for two-dimensional synthesis and outputs digital image signals corresponding to the fluorescence image formed by the radiation, and an image reproduction means which inputs the digital image signals from the processing means to reproduce the fluorescent image formed by the radiation, wherein the radiographic image conversion screen is a radiographic image conversion screen as defined in any one of the above (9) to (12).
(14) The radiograph-forming device according to the above (13), wherein the digital image signals are subjected to an image processing by means of an image processing means, and then input into the image reproduction means.
(15) The radiograph-forming device according to the above (13) or (14), wherein the peak of the spectral sensitivity of the photoelectric conversion elements is in a wavelength region of from 450 to 650 nm.
(16) The radiograph-forming device according to any one of the above (13) to (15), wherein each of the photoelectric conversion element is a photodiode.
(17) The radiograph-forming device according to the above (16), wherein the photodiode is an amorphous silicon photodiode or a silicon photodiode having a peak of the spectral sensitivity within a wavelength region of from 500 to 600 nm.